Cyanidation is a process used in the mining industry where metals are leached from ores into a weak solution of sodium cyanide. Once the metals are leached from the ores, the metals such as gold and silver are precipitated and separated from the sodium cyanide solution. A relatively large portion of the sodium cyanide solution is recirculated back to the cyanidation process to take advantage of the leaching potential for the metals. It is, however, necessary to blow down a portion of this barren solution to avoid the buildup of unwanted metals and anions. Eventually the blowdown is sent to a tailings pond or other holding area for removal of metals and cyanide prior to discharge. The increasing cost of cyanide, strict environmental regulations, and a need to insure sustainable operations have led to increased interest in cyanide recovery.
A number of processes for recovering cyanide from barren solutions have been developed. Most of the processes require that the recoverable cyanide is first converted to the highly toxic hydrocyanic acid (HCN). Handling of this compound presents concerns from a health and safety perspective. An acidification, volatilization, and re-neutralization (AVR) process has been used in the past for cyanide recovery. However, this process has several drawbacks: 1) it recovers only free cyanide, 2) it cannot recover cyanide from its complex forms, and 3) because of the high cyanide to air ratio, the CAPEX and OPEX of the process are high. Furthermore, since the presence of HCN in the stripping column is potentially hazardous, the columns must be leak proof.
Besides the AVR process, several other processes have been investigated to recover cyanide from weak acid dissociable (WAD) metal complexes (copper, zinc, and nickel cyanide). For example, the Sulphidization, Acidification, Recycling and Thickening (SART) process was developed, but has not been operated yet in a full-scale plant so its reliability is unknown. Laboratory and pilot-scale systems have been used to evaluate the applicability of ion exchange (IX), adsorption onto granular activated carbon, and adsorption onto activated alumina. However, information pertaining to the full-scale installation of these processes is not available yet. Therefore, an efficient cyanide recovery technology that can minimize the volatilization of HCN into the environment is needed.